Analysis of mouse mammary tumor virus (MMTV) induced premalignant mammary epithelial outgrowth (HOG) lines and mammary tumors led to the identification of common integration sites (CIS) restricted to 35 loci. Using parallel expression analysis from the matched tumors, we determined that within these loci members of the Wnt(n=2), Fgf (n=4) and Rspo(n=2) gene families plus two linked genes (Npm3 and Ddn) and three unlinked genes (Notch4, Sfmbt2 and Pdgfra) were frequently activated in tumors induced by MMTV irrespective of the mouse strain or the strain of virus. A second group of 13 CIS occur at a low frequency (2-5 observations) in mammary HOGs or tumors and are mouse or virus strain specific. The expression levels of 20 of the human genetic homologues of MMTV-CIS associated genes are deregulated and/or mutated in human breast tumors.